Polyethylene terephthalate (hereunder abbreviated as “PET”) fibers are widely used as constituent materials of implantable medical equipment such as stent graft fabrics and artificial blood vessels.
Stent grafts are discussed here. Conventional treatment for aortic aneurysm has included artificial blood vessel replacement using e-PTFE or PET artificial blood vessels, but because such techniques involve large-scale surgical operation such as thoracotomy or laparotomy they are highly burdensome for the body and are limited in their suitability to the elderly or patients with complications, while the economical burden for patients and medical facilities is also significant due to the need for long-term hospitalization. On the other hand, transcatheter intravascular treatment using “stent grafts” (a method of treatment in which a narrow catheter having a stent graft compressively inserted therein is introduced through the artery at the base of the foot, and the stent graft is opened and fixed at the site of aneurysm, whereby blood flow into the aneurysm is blocked and burst of the aneurysm is prevented), fitted with a tubular fabric (hereunder referred to as “stent graft fabric”) in a spring-like metal known as a stent, does not involve thoracotomy or laparotomy, and therefore in recent years its application has been rapidly increasing since physical and economical burden is reduced.
However, because current stent grafts cannot be folded small and can only be inserted into catheters with thick diameters, in many cases they cannot be applied to females and Asians such as Japanese, which have narrower arteries. Given these circumstances, there is increased need for reducing the diameters of stent grafts, and also demand for stent grafts with maximum inner diameters of 50 mm to be insertable into catheters of 18 French (inner diameter of 6 mm) and smaller, for the thorax, for example.
Narrowing of stent grafts can be achieved by modifying the shape of the stent or the filament diameter of the metal, but since stent grafts are basically fixed to the affected area by a system of pressing against the vascular wall by the expanding force of the metal, there has been a limit to the improvement that can be achieved with expanding force by narrowing the filament diameter. On the other hand, narrowing can also be achieved by reducing the thickness of the stent graft fabric. A stent graft fabric employs an e-PTFE film or a PET fiber woven fabric or knitted fabric, and when the thickness of an e-PTFE film is reduced there is a risk of thinning and drawing and burst of the film with time due to expanding force by the stent or blood pressure, and therefore the degree to which e-PTFE can be reduced in thickness is limited. In order to reduce the thickness of the stent graft fabric, therefore, it is effective to reduce the thickness of the PET fiber fabric, and for this purpose it is necessary to reduce the total fineness and single fiber fineness of the PET fibers composing the fabric, or in other words, to use superfine fibers.
The following types of superfine PET fibers have been known in the prior art.
(a) Sea-island Superfine PET Fibers
Sea-island superfine PET fibers are spun as undrawn filaments having a sea-island cross-section, from a plurality of different polymer components such as PET as the island component and copolymerized PET or polyamide as the sea component, using a melting process, and the undrawn filaments are drawn to a draw ratio within the natural drawing range for PET which is the island component, and then the sea component is removed by dissolving with a solvent.
(b) Polymer Blend Superfine PET Fibers
Polymer blend superfine PET fibers are obtained by melt spinning of a mixture of two or more different polymer components that have different solubilities and are poorly compatible, spinning sea-island fibers with one of the polymers microdispersed inside the other, and after spinning, the sea component is removed by dissolution with a solvent, as in (a) above.
(c) Direct-spun Superfine PET Fibers
Direct-spun superfine PET fibers are obtained by melt spinning a PET polymer alone, to obtain undrawn PET fiber, and drawing it.
Because sea-island and polymer blend superfine PET fibers are obtained by removing the sea component polymer by dissolution with a solvent as described above, the solvent or sea component polymer, or even the hydrolyzable monomer of the sea component, can residually adhere onto the superfine PET fibers, and potentially elute into the body. This is a crucial problem from the viewpoint of biological safety, as a material for implantation into the human body. In addition, since sea-island and polymer blend superfine PET fibers have the sea component removed by dissolution with a solvent after being formed into a fabric, gaps form in the textile structure, and when it is used as a stent graft fabric, for example, endoleak can potentially occur at those locations.
On the other hand, PTLs 1 to 3 disclose direct-spun superfine PET fibers obtained by direct melt spinning methods. Such direct-spun superfine PET fibers do not carry the risk of residue, and can be considered highly safe materials for biological use. However, when conventional direct-spun superfine PET fibers are compared to PET fibers of normal thickness (hereunder referred to as “regular PET fibers”), their strength has been found to be reduced. This is because in a conventional direct melt spinning method it is necessary to minimize the melt viscosity of the polymer until it reaches the spinneret, in order to accomplish continuous stable spinning, and since a starting polymer with a low polymerization degree is used for this purpose, lower strength has been exhibited compared to regular PET fibers. In the case of superfine fibers, non-homogeneity of cooling of the melting filaments discharged from each spinneret results in considerable effects of fiber size variation between filaments or in the fiber axis direction and results in a structure with poor expression of strength, with the tensile strength of the direct-spun superfine PET fibers described in PTLs 1 to 3 having been at most about 3 cN/dtex.
In the case of a stent graft, the high expanding force of the stent (spring-like metal) reaches the fabric when the stent graft is opened from a catheter at the affected blood vessel. The stent graft is also exposed to the conditions of the load of normal blood pressure. Using the superfine PET fibers with low strength described in PTLs 1 to 3, in consideration of the requirement for a stent graft fabric to have sufficient strength to withstand the high expanding force of a stent (spring-like metal) and to withstand the load of blood pressure, and specifically a burst strength of 100N or greater based on ANSI/AAMI, such fibers having a tensile strength of about 3 cN/dtex cannot form a fabric that meets this requirement.
Furthermore, in the case of a stent graft being used as a substitute material for a blood vessel, lack of endoleak is an essential feature and the woven texture must be highly dense, in the case of weaving, for example, in order to form a fabric with no endoleak. Nevertheless, the direct-spun superfine PET fibers described in PTLs 1 to 3 produce yarn breakage or fluff during processing even when forming a sheet-like woven fabric, making it difficult to achieve high density, and in particular it has been extremely difficult to realize high density with tubular seamless fabrics.
For these reasons, it has not yet been possible to obtain excellent biological safety for the constituent fibers of fabrics for low profile stent grafts, or to obtain superfine polyester fibers that are both fine and strong. Moreover, it is currently the case that no fabric has been obtained that exhibits both thinness and strength satisfying the requirements for low profile of stent grafts.